A. Field of the Invention
This application relates to liquid crystal devices, particularly devices employing ferroelectric liquid crystals.
B. U.S. Pat. No. 4,367,924
In U.S. Pat. No. 4,367,924 (hereinafter "said patent"), the contents of which are incorporated herein by reference, a liquid crystal electro-optic device is described employing a chiral smectic C or H ferroelectric liquid crystal. In that device the liquid crystal is disposed between parallel plates with the planar smectic layers normal to the plates (see said patent, FIG. 2). These smectics are characterized by an average molecular long axis direction, indicated by the molecular director, n, which is constrained, in equilibrium, to make some temperature dependent angle, .PSI..sub.o, with the normal to the layers, but which is free to take up any value of the angle .phi. which gives the orientation of n about the layer normal. Typically .PSI..sub.o, which is a property of the bulk smectic, is in the range from 0.degree. to about 45.degree.. The ferroelectric polarization, P, reorients with n, always remaining locally normal to n and lying parallel to the plane of the layers, as shown in FIGS. 1 and 2 of said patent.
In the device described in said patent, the plates were treated so that the molecules near the plates would adopt an orientation having the average molecular long axis direction parallel to the plane of the plates but free to adopt any orientation within that plane. That is, the molecular director, n, is constrained at the surface to lie in the surface plane. This condition, when combined with the additional constraint that the director make the angle .PSI..sub.o with the normal to the layers (see said patent, FIG. 2), leads to a geometry in which, if the plates are sufficiently close together, the intrinsic helical configuration of n which is present in the bulk will be suppressed, leaving two surface stabilized states of the molecular orientation configuration, each having the ferroelectric polarization normal to the plates but in opposite directions (see said patent, FIG. 2). Devices such as this, which employ surface interactions to stably unwind the spontaneous ferroelectric helix, will be referred to as Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) devices.
The device of said patent exhibits several novel features which distinguish it from other liquid crystal devices:
(1) Optic axis rotation about the sample normal - A ferroelectric smectic in this geometry behaves optically as a biaxial slab with the optic axes nearly along the director orientation. The biaxiality is generally weak, so the behavior is essentially uniaxial with the uniaxis along the director. The effect of switching is to rotate the uniaxis about the normal to the surface through an angle of twice the tilt angle .PSI..sub.o. This is the only liquid crystal parallel-plate geometry allowing a rotation of the uniaxis of a homogeneous sample about the surface normal.
(2) Strong-weak boundary conditions - Another unique feature to be noted is the nature of the required boundary condition. In order to obtain bistability, boundary conditions which constrain the molecules to be parallel to the plates but allow several or continuous orientations about the normal to the plates are required. The device of said patent is the first liquid crystal electro-optic structure to employ such a combination of strong and weak boundary conditions. A consequence of this feature and an essential property of the structure is that the director at the surfaces is switched between stable surface orientation states as an intrinsic part of the overall switching process. The SSFLC is the first liquid crystal electro-optic structure wherein switching between stable surface states has been demonstrated and the first case in which ferroelectric liquid crystal domains have been made to appear.
(3) A significantly higher switching speed - As a result of having the helix unwound, it is the first ferroelectric liquid crystal device to achieve the minimum, intrinsic response time for molecular reorientation to a changing electric field, since, with the helix unwound, bulk reorientation can occur without the motion of topological defects in the orientation field.
C. Achieving Layer and Director Alignment
An indispensable requirement necessary to make a practical electro-optic device using the surface stabilized ferroelectric liquid crystal geometry is to achieve, in the ferroelectric smectic phase, both the desired director boundary condition and the desired layer orientation in a uniform fashion over the entire active device area. Boundary conditions influencing the director orientation at the surface are established by specific surface preparations. Possible boundary conditions and their properties in the surface stabilized ferroelectric liquid crystal geometry are discussed in detail in the next section. Uniform layer orientation, on the other hand, must be established by some specific step appropriately controlling the growth or arrangement of the smectic layers in the process of fabricating the liquid crystal cell.
Excluding the geometry having the liquid crystal layers parallel to the plates, which is not of relevance to this application, there are only three ways demonstrated in the art of achieving uniform layer orientation of a smectic C or a tilted smectic crystal. The first are the well-known anisotropic surface treatment techniques of rubbing or oblique SiO evaporation, combined with the smectic A - C transition, as reviewed by K. Kondo et al in the Japanese Journal of Applied Physics, Volume 20, pp. 1773-1777, 1981. The other two are the combination of external shear and the smectic A - C transition described in said patent, and the combination of magnetic field and the smectic A - C transition, also described in said patent and since reported by K. Kondo et al, op. cit. Using the nematic to smectic C transition has not proven successful in achieving homogeneous layer alignment, since even the strong surface planar alignment provided by oblique evaporation of silicon monoxide, which fixes the director orientation in space, produces two different layer orientations in distinct regions, as has been demonstrated by M. Brunet, Le Journal de Physique, Volume 36, pp. Cl, 321-324, 1975, and G. Peltzl et al., Molecular Crystals and Liquid Crystals, Volume 53, 167-180, 1979.
All of the above-mentioned treatments involve ordering in one phase and cooling into the smectic C phase. They involve a combination of processes and are thus complicated compared to, for example, the alignment process for twisted nematic cells, which requires only surface treatment. It would be desirable to have available processes for alignment of layers in ferroelectric smectics which involve only surface treatment (and perhaps the use of a phase transition) which provide controllable surface orientation characteristics for the director in the ferroelectric smectic phase. In Section X E, techniques are discussed for layer orientation of ferroelectric liquid crystals and a novel method is introduced involving two kinds of boundary condition, one acting upon the director, the other upon the layer.
D. Ferroelectric Liquid Crystals
In said patent, an electro-optic device employing a ferroelectric smectic C or H liquid crystal was described. The features of these ferroelectric phases essential to the operation of the device are: (1) they are smectics in which the rod-shaped molecules are arranged into layers with the director tilted at some angle relative to the normal to the layers; (2) the molecules are chiral, producing, according to the arguments of Meyer et al (Le Journal de Physique, Volume 36, pp., L69-71, 1975), a bulk ferroelectric dipole moment, P, normal to n. The chiral compounds discussed in the original application, DOBAMBC and HOBACPC, have several tilted smectic - and therefore ferroelectric - phases, two of which were identified at the time of the application: the smectic C phase over some temperature range and, at lower temperatures, a phase which we described as smectic H in accord with the identifications made by the group that synthesized the compounds (P. Keller et al, Le Journal de Physique, Volume 37, pp. C3-27, 1976). However, other data (J. Doucet, et al, Le Journal de Physique, Volume 39, pp. 548-553, 1978) suggested that there are at least two phases below the smectic C in HOBACPC, a smectic F (now called smectic I) adjacent to the C, and a smectic H at lower temperatures. More recent heat capacity studies by the inventors confirm the latter data. The nomenclature of the various tilted smectic phases has been subject to some changes in the last two years and both their crystallographic identification and nomenclature is in a state of considerable flux.
The presently adopted distinction among the five ferroelectric smectic phases now identified with certainty--smectics C,F,G,H,I--is as follows. The smectic C phase is the most fluid, having normal liquid state order within a given layer, i.e., local positional order involving only a few molecules. The smectic F and I phases have considerably more order in a given plane, with typically hundreds of molecules grouped into local quasi-crystalline regions. This leads to an orientational viscosity of the smectic F and I which is about 100 times that of the smectic C. The smectic F and I are distinguished by the different orientation of the tilt direction relative to the local crystal lattice direction. The smectic G and H phases are much more strongly ordered, with nearly long range (quasi-crystalline) translational ordering in a given layer, and very high viscosities. New subdivisions of the G and H classes, necessitating the further denominations J and K, have recently been proposed.
However, the crystallographic details of internal ordering add nothing new in principle. All chiral tilted smectics are ferroelectric and although the original application discussed specifically the smectic C and H phases (now identified as C and I in HOBACPC and C and F in DOBAMBC), we point out here that devices like that of said patent or like those to be described in this application may employ any o these ferroelectric phases and operate in essentially the same fashion. There will be qualitative features of the various phases that will dictate which to use in a particular situation. For example, as one proceeds from the least to the most strongly ordered, the electro-optic switching may become slower but with improved stability (memory, threshold) characteristics of the switching. Also, as the correlation between smectic layers grows stronger, the helix pitch will increase (becoming infinite in the limit of sufficiently strong ordering), allowing thick samples to be switched bistably.